Apparatus for protecting piezoelectric crystals from overload



June 27, 1939- J. M. wou-'SKILL APPARATUS FOR PROTECTING PIEZOELECTRIC CRYSTALS FROM OVERLOAD Filed April 6, 1938 2 sheets-sheet 1 4% E/-EEI IN V EN TOR.

Patented June 27, 1939 UNITED STATES PATENT OFFICE APPARATUS FOB PROTECTING PIEZOELEC- TRIC CRYSTALS FROM OVERLOAD Erie, Pa.

Application April 6, 1938, Serial No. 200,529

11 Claims.

This invention relates to piezo-electric devices in general and more particularly to devices for protecting piezo-electric crystal elements from being overloaded. In circuits Where a piezoelectric crystal is used for controlling or stabilizlng the frequency of an oscillator, the crystal may become overloaded and mechanical vibration become so intense as to shatter or rupture the crystal.

It is an object of this invention to adapt an element to be used in conjunction with a crystal in which the overloading is eliminated, and safe loading is automatically governed or adjusted.

Another object of this invention is to provide l a holder, such that the regulating device is connected internally across the crystal, so that the crystal cannot be used Without protective action of this controlling device.

Another object of my invention is to increase the harmonic content of the oscillations produced by a crystal controlled oscillator by virtue of the non-linear resistance characteristic of the controlling device.

Another object is to provide a controlling element with characteristics such that the crystal is readily set into oscillation.

Other objects of my invention will be disclosed in the following description.

In oscillator circuits where a piezo-electric device is used to control the oscillation frequency or to stabilize the frequency, the radio-frequency current through the crystal may become excessive, and the strain on the crystal become so great as to shatter it. At resonance, this strain is naturally greatest, and may be excessive with a fairly low radio-frequency voltage across it. This voltage can either be impressed on the crystal, and the crystal driven at its natural period, or the crystal may be used to drive an oscillator tube; in the latter case, the energy fed back to the crystal from the output by way of the grid plate capacity within the tube, is apt to reach such proportions as to cause arcing in the crystal, and possible fracture.

Many schemes have been devised to protect quartz crystals from developing this excessive voltage or passing an excessive radio-frequency current (up to the yielding point the radio frequency voltage and current bear a linear relation to each other). These included fuses, lamps,

ballast lamps, and neon lamps. All these methods have their attendant faults, because of the fact that the limiting or governing action is not rapid enough to prevent the r-f voltage from building u up and breaking the crystal.

(Cl. Z50-36) A material sold to the trade under the name of Thyrite, having the required characteristic, is suitable for this purpose. This material and the manner of processing is described in United States Patent No. 1,822,742 to K. B. McEachron. Due to its non-linear resistance characteristic with current (both radio frequency and direct current) the resistance decreases very rapidly with increasing current through the element.

It is of course. obvious that other resistance materials beside that sold under the name Thyrite may be employed in accordance with this invention as long as the resistance element used possesses the desired characteristics.

Further details of this invention will be set forth in the following specification and the drawings in which, briefly, Fig. 1 illustrates a schematic diagram of an oscillation generator circuit arrangement showing my invention; Fig. 2 is another schematic diagram showing a modified form of circuit; Figs. 3 and 4 illustrate characteristics of the resistance material that I employ in accordance with this invention; Fig. 5 is a curve showing variation of the current flowing through the crystal as the oscillator tuning condenser is adjusted; and Figs. 6 and 7 are sectional views for illustrating the resistance element incorporated in the crystal holder.

One type of circuit in which a crystal is employed is shown in Fig. 1. In this case the resistance element 5 is of silicon carbide, Tl-write" processed as described in Patent No. 1,822,742 or similar material and is connected across the contact members 4 associated with the crystal plate 2. These electrodes or contact members 4 are connected to the grid electrode 6 and cathode 'I of the tube 9. The anode 8 is connected to the oscillatory ,circuit Il, which includes an inductance unit I2 and a variable condenser I0. A source of anode current supply I4 shunted by the by-pass condenser I3 is connected to the oscillatory circuit Il and the cathode l.

The operation of the circuit in Fig. 1 is as follows: The condenser I0 is A'adjusted to tune the oscillatory circuit l I to the crystal frequency and the crystal plate 2 impresses electric charges in the form of oscillations on grid 6 of the tube 9, corresponding to the frequency to which the circuit Il is tuned. The d-c grid current to the tube flows through the Tlryrite element 5. As the feedback capacity is increased, and the crystal 2 driven harder, the r-f voltage across the Thyrite element 5 will be increased, and as a result, the resistance of the element 5 Will decrease. 'Ihis decrease in resistance will in turn reduce the voltage across the crystal, and a governing action will result, the circuit finally stabilizing at some intermediate safe value. Not only is the r-f voltage generated by the crystal eifective in reducing the resistance of the Thyrite element with excessive excitation, but

the d-c grid current in the tube 9, which increases with excitation, will also be effective in reducing the resistance of this element still further. In the particular circuitshown in Figure 1, then, both the r-f voltage and the d-c grid current are effective in producing a governing action on the voltage developed across the crystal.

Typical curves showing the r-f voltage resistance characteristics are shown in Figs. 3 and 4. As shown in the curve in Fig. 3, the resistance decreases rapidly with increasing r-f voltage across the element 5. With an r-f voltage across the crystal, as well as the Thyrite unit 5, of about 200 volts, the resistance of the unit is about 50,000 ohms, or any value for which it was designed, which is a nominal value of grid resistance for an oscillator tube. This value, however, can be varied over wide ranges by processing the element accordingly. The low resistance characteristics of an element of this type is shown in Fig. 4.

Figure 5 shows curves of crystal current plotted against tuning capacity in tank circuit II. 'I'he amplitude of oscillation of the crystal naturally increases as the resonance point of the oscillatory circuit Il approaches the natural frequency of the crystal.

Curve I in Figure 5 shows how the crystal current (or amplitude of oscillation) increases with a fixed carbon resistor connected across the crystal in place of the Thyrite element 5. 'I'his curve is characterised by the steepness of the slope at its high end.

Curve 2 in Figure 5 shows the variation of the crystal current with a Thyrite element connected across the crystal as in Figure 1. 'Ihe curve illustrates how the governing action of the Thyrite has decreased the crystalcurrent and reduced the slope of the curve at its highest point.

The 'Ihyrite element resistance was equal approximately to that of the fixed carbon resistor when 50 volts r-f was impressed across the element.

No loss of power output from the oscillator was noted when the xed resistor was replaced with the Thyrite.

Another type of circuit is illustrated in Figure 2, in which the resistance of the Thyrite element 5 is governed only by the r-f voltage developed by the. crystal, the d-c grid current being shunted by a parallel path through radio-frequency choke 3 and resistor I5. 'I'he d-c grid current cannot iiow through 'I'hyrite element 5 due to blocking condenser I; and hence, it is not effective in decreasing the resistance of this element. It has been found that by combining both the effects of the d-c grid current and r-f voltage as in Figure 1, the shunting and governing action of the Thyrite element is most pronounced. In addition to these desirable characteristics, the 'I'hyrite itself has an impedance of several megohms when a very low voltage is impressed across it and as a result provides ideal conditions for starting the crystal into oscillation. As sion as oscillations build up, a radiofrequency voltage is developed, and the resistance immediately drops to a nominal value, for example 50,000 ohms, or any value for which the Thyrite was designed. By using this material, it is practically impossible to build up an r-f voltage across any crystal exceeding several hundred volts. Inasmuch as the resistance 0f the Thyrite decreases at such a rapid rate, the action of the material is practically instantaneous, and even a very rapid surge in r-f voltage decreases the resistance of the element.

The Thyrite element 5 may be mounted any- Where in the oscillator cabinet as would any other component, or it may be mounted in the crystal holder I6 which is provided with plugs I1 and I8 as shown in Figures 6 and '7. The element is connected to the plugs I'I and I8 and is thus always in circuit to protect the crystal2 from excessive voltage whenever the crystal is plugged into a circuit carrying a radio frequency or other current. A spring I9 is positioned adjacent to the element 5 and is employed to hold the crystal and crystal electrode assembly against the top 20 of the holder. This spring is also connected to the plug I'I to provide electrical connection to the lower crystal electrode. 'I'he top electrode is connected to the plug I8 by the bolt 2I and conducting member 22.

The element 5 is shaped in the form of a rod in order to keep the radio-frequency capacity added across the electrodes 4, low. The crystal holder housing I6 is constructed to receive the resistance element 5 in the bottom of the cavity formed therein. In this way the crystal 2 may be readily removed from the holder housing without necessitating removal of the resistance element inasmuch assthe kcrystal and electrodes are adjacent to tha noverf 20 which is held on the housing by suitable bolts. be employed as the top electrode and the construction simplied tovthis extent.

It will be observed that various modifications may be made in the details of my invention without departing from the spirit and scope thereof and therefore I do not desire to limit the invention to the exact details shown and described except insofar as they are defined by the claims.

I claim:

1. In an oscillation generator of the type employing a piezo electric crystal frequency controlling or stabilizing element, the combination of an oscillation generating tube having a work circuit connected thereto, a piezo electric crystal having predetermined frequency characteristics connected to said tube, a continuously conductive resistance element having a non-linear resistance-voltage characteristic, means for connecting said resistance element across said crystal,

the characteristics of said resistance element bef ing such that the high frequency voltage across said crystal is continuously maintained at predetermined values through the loading action of said resistance element.

2. In an oscillation generator of the type employing a piezo electric crystal. frequency con'- Where desired the cover 20 may y trolling or stabilizing element, the combination of an oscillation generating tube having a work circuit connected thereto, a piezo electric crystal having a predetermined frequency characteristic connected to said tube, a continuously conductive resistance element having a non-linear resistance-voltage characteristic, means for connecting said resistance element across said crystal, said resistance element having resistancevoltage characteristics such that the greater the voltage across said resistance element ythe lower the resistance of said element whereby the shunting eiect of said element is limited sub.- stantially to the high values of high frequency potentials harmful to the operation of said crystal.

3. In apparatus as set forth in claim 1, said resistance element consisting substantially of silicon carbide.

4. In apparatus as set forth in claim 1, a condenser connected between said resistance element and said piezo electric crystal.

5. A piezo electric crystal holder comprising: a housing having a cavity for receiving a piezo electric crystal, electrodes for contacting the crystal, terminals attached to said housing, connections *betweenl said electrodes and said terminals, a resistance element positioned in said cavity below said crystal electrodes, said resistance element having voltage-resistance characteristics such as to protect the crystal positioned between said electrodes from electrical overload, spring means for holding the lowermost of said electrodes away from said resistance element, and connections between said resistance element and said terminals.

6. A piezo electric crystal holder comprising: a housing, a piezo electric crystal having predetermined characteristics positioned in said housing, electrodes for said crystal, terminals attached to said housing, connections for connecting said terminals to said electrodes, and an elongated rod-like resistance element having a low highfrequency capacity connected as a protective element across said crystal electrodes, said resistance element having a substantially hyperbolic resistance-voltage characteristics such as to substantially instantaneously protect said crystal from electrical overload.

7. A protective device for piezo electric crystals comprising: a continuously conductive substantially silicon carbide resistance element, means for connecting said element across a piezo electric crystal for instantaneously reducing excessive high frequency voltages tending to build up across the crystal.

8. In piezo electric crystal controlled oscillation generators and like apparatus, a piezo electric crystal, electrodes for said crystal, said crystal having electrical oscillations of various potentials across the electrodes thereof, a continuously conductive loading resistance element having a non-linear resistance-voltage characteristic, said resistance element consisting of a conducting member and connecting means attached thereto for impressing excessive potentials generated across said piezo electric crystal upon said resistance element to regulate the potential across said crystal continuously, said resistance element having characteristics such that the resistance thereof varies gradually with gradually varying high frequency voltage or substantially instantaneously with instantaneously varying voltage without abruptly decreasing its resistance over j any portion of its resistance-voltage characteristic to such an extent as to virtually short circuit said piezo electric crystal.

9. In piezo electric crystal controlled oscillation generators and like apparatus, a piezo electric crystal, electrodes for said crystal, said crystal having electrical oscillations of various potentials across the electrodes thereof, a continuously conductive resistance loading element having a non-linear resistance-voltage characteristic, said resistance element consisting of a conducting member and connecting means attached thereto for impressing excessive potentialsv generated across said piezo electric crystal upon said resistance element to regulate the potential across said crystal continuously, said resistance element having substantially instantaneously responsive characteristics such that the resistance thereof decreases as the high frequency voltage across said piezo electric crystal increases whereby said resistance element tends to maintain the high frequency voltage across said piezo electric crystal at safe values to protect said crystal.

10. In piezo electric crystal controlled oscillation generators and like apparatus, a piezo electric crystal, electrodes for said. crystal, said crystal having electrical oscillations of various potentials across the electrodes thereof, a continuously conductive resistance element composed substantially of silicon carbide and having a nonlinear substantially hyperbolic resistance-voltage characteristic, said resistance element consisting of a conducting member and connecting means attached thereto for impressing excessive potentials generated across said piezo electric crystal upon said resistance element to regulate the potential across said crystal continuously, said resistance element having characteristics /such that the resistance thereof decreases as the high frequency voltage across said piezo electric crystal increases whereby said resistance element tends to maintain the high frequency voltage across said piezo electric crystal at safe values to protect said crystal.

l1. In piezo electric crystal controlled oscillation generators and like apparatus, a piezo electric crystal, electrodes for said crystal, said crystal having electrical oscillations of various potentials across the electrodes thereof, a continuously conductive resistance element composed of silicon carbide and having a non-linear substantially hyperbolic resistance-voltage characteristic, said resistance element consisting of a conducting member and connecting means attached thereto for impressing excessive potentials generated across said piezo electric crystal upon said resistance element to regulate the potential across said crystal continuously, said resistance element having characteristics such that the resistance thereof decreases gradually with increasing high frequency voltage withoutabruptly decreasing its resistance over any portion of its resistance-voltage characteristic to such an extent as to virtually short circuit said piezo electric crystal.

JOHN M. WOIFSKIIL. 

